The River Great Ouse/ˈuːz/ is a river in the United Kingdom, the longest of several British rivers called "Ouse". From Syresham in central England, the Great Ouse flows into East Anglia before entering the Wash, a bay of the North Sea. With a course of 143 miles (230 km), mostly flowing north and east, it is the fourth-longest river in the United Kingdom. The Great Ouse has been historically important for commercial navigation, and for draining the low-lying region through which it flows; its best-known tributary is the Cam, which runs through Cambridge. Its lower course passes through drained wetlands and fens and has been extensively modified, or channelised, to relieve flooding and provide a better route for barge traffic. Though the un-modified river probably changed course regularly after floods, it now enters the Wash after passing through the port of King's Lynn, south of its earliest-recorded route to the sea.